When it comes to data center design, power distribution architecture is arguably one of the most important aspects to be considered. Data center owners and operators take every opportunity to improve power efficiency and reduce operating cost. There’s been a debate over which power distribution architecture, namely Alternating Current (AC) or Direct Current (DC), is the most efficient for data centers. This article will walk you through the basics of these two data center power systems and how they compare to each other.
Alternating current (AC) power is a standard electricity format that comes from a power plant. The direction of the current reverses, or alternates periodically, 60 times per second (in the U.S.) or 50 times per second (in Europe).
In an AC power data center, the power is distributed to the facility at 600V AC or 480V AC. Then this power is stepped down to 208V AC or 120V AC via transformers for distribution to racks for use by servers and other IT equipment.
An uninterruptible power supply (UPS) and energy storage systems such as batteries are used as power backup for power interruption and disturbances. The incoming AC power has to be converted to DC for storage. When power interruption occurs, power backup converts stored DC power to AC power. The power is further transmitted to PDUs and then to servers and IT equipment placed in racks.
Direct current (DC) power is an electricity format that comes from batteries and sources such as solar cells and batteries. The positive and negative terminals are always, respectively, positive and negative. So the current is linear and direct, and always flows in the same direction and does not oscillate between positive and negative terminals.
DC power data centers get medium-voltage AC power from the Grid. It then gets converted to DC using rectifiers. DC power from these rectifiers goes to the battery distribution circuit breaker bay (BDCBB) which transmits the power to servers placed in racks directly or through fuse alarm panels.
During power interruptions or disturbances, the charged batteries are used for power backup. But unlike with AC power, conversion is not required in this case.
As mentioned, the main difference between AC and DC power is the direction the electrons flow. This distinction leads to all other differences between these two power distribution systems.
In AC power, the current changes directions periodically, and the voltage is easily transformed. This makes it easy to transport over a longer distance of several miles as compared to DC power.
However, every AC to DC conversion results in power losses and heat generation. The fewer conversions the power supply undergoes, the lower the losses and the less heat generated. Greater efficiency leads to lower costs, both in capital and O&M. In this sense, a DC power data center tends to be more energy-efficient as a AC power data center involves more AC to DC conversions.
Before jumping to conclusions on which power distribution system is better, we have to examine the pros and cons of both.
Advantages of AC Power
Standardized source: AC power is a standardized source of primary voltage electricity. It is the form in which electric power is delivered to residences, businesses, and also data centers. Today our electricity is still predominantly powered by alternating current.
Less power loss over long distances: When transmitting power over long distances, such as from a power plant to an urban area, a very high voltage of 600,000 V (volts) is used to improve transmission efficiency. This is because power loss is much lower when power is transmitted at high voltage.
Easy to transform: Compared to direct current, alternating current can be easily transformed using transformers, making it more suitable for power supply as infrastructure.
Easy to shut down: AC power is easy to shut down while power is being supplied bacause the timing at which the voltage drops to zero comes periodically.
Disadvantages of AC Power
Requires a higher voltage than the target voltage: AC requires a higher voltage than the target voltage for the required amount of heat because the voltage value is always changing, and there are times when the voltage goes to zero.
Affected by coils and capacitors: Coils and capacitors generate voltages that cause the current to flow in the opposite direction of the current direction, causing the current in the circuit to advance or lag.
Multiple conversions: AC power has to be converted to DC power for storage, and the stored DC power has to be converted back again to AC power. This involves multiple conversions in data centers, which can translate into lower efficiency and higher cost.
Pros & Cons of DC Power
For more detailed info about DC power, please refer to Direct Current (DC) Power: Is It the New Normal for Data Centers?
DC power has enormous potential. Benefits include considerable energy savings, as well as significant savings on operating and infrastructure costs, physical space, and the amount of time required for installation and maintenance.
Compared to AC power, DC power distribution system is less complex, uses significantly fewer components, and takes less space. Plus, the quality of DC power is better than that of AC. Finally, it’s much easier to integrate renewable energy sources such as solar, fuel cells, or wind turbines.
However, DC power systems are still missing an agreement on a standard for electrical voltages and electrical connectors. There is also a limited number of infrastructure on the market that runs on DC power supplies, such as DC air conditioning and fire protection systems.
From the discussion above, it is not hard to see that vast amounts of unused electricity simply disappear in AC power data centers. They are lost to tedious and continuous conversion and transformation processes. To cope with the significant power loss, some come up with ideas such as installing a power supply that allows AC power use but also allows connecting a battery that will work as a UPS system if the AC power fails. But it works well only for smaller sites, and doesn’t scale efficiently for larger sites with high customer density.
That's probably why the industry has a growing interest in the notion of moving to direct current for power distribution within the data center, coupled with all the benefits that DC power offers. However, switching the prevailing AC power supply to DC, which would effectively eliminate a significant percentage of power losses, will require a massive shift. It may not be possible in the near future for data centers to switch the existing AC system to full DC power systems and batteries, and use DC powered equipment at all sites.
Though the global installed base of DC data centers today only constitutes a tiny fraction of the industry, the case for DC power still remains compelling. If more data center owners and operators were to make a substantial investment in DC, it could help drive the development of DC standards and DC equipment such as DC servers, networking switches, etc., to take DC power distribution system to the next level.